Purple benzene

The insoluble salt begins to dissolve when you add a third-party agent, giving a pink-colored solution

Scientific name: Interaction between potassium permanganate and 18-crown-6 leads to the formation of a complex which is soluble in benzene

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Purple benzene

by ChemToddler

Purple Benzene- exploring crown ethers

by Reactionphile

properties of ethers and crown ethers

by Khan Academy Organic Chemistry

Safety

Wear eye protection goggles. Use gloves.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

KMnO4 + [C2H4O]6 → KMnO4*[C2H4O]6

K+ + [C2H4O]6 → K+*[C2H4O]6

Step-by-step instruction

WARNING! This experiment is dangerous! You should NOT perform this at home. ONLY carry out this experiment, if you are a trained chemist, and you understand local safety and legal requirements, which are required to perform such experiments

  1. Put a stirbar in a beaker. Put the beaker on a magnetic stirrer.
  2. Pour 100 ml of benzene into the beaker. Turn on the magnetic stirrer.
  3. Add 2 g of potassium permanganate.
  4. Make sure that the potassium permanganate is insoluble in benzene, and the benzene is colorless.
  5. Pour few hundred milligrams of 18-crown-6 into the beaker.
  6. After some time, the solution becomes purple, and potassium permanganate slowly dissolves.

Scientific background

Crown ethers are heterocyclic compounds containing more than 11 atoms in their cycles, at least four out of them are heteroatoms linked through an ethylene bridge.

Crown ethers are viscous liquids or crystalline substances, soluble in most organic solvents and poorly soluble in water. Their properties depend on the chemical nature of the functional groups and heteroatoms in the ring.

Crown ethers form stable lipophilic complexes with metal cations. The metal cation is included in an intramolecular cavity of the crown ether and retained by ion-dipole interactions with heteroatoms. Complexes which have the size of the metal ion is close to the size of the cavity of the crown ether molecule are most stable.

Since the diameter of the cavity in a molecule of 18-crown-6 is 2,6-3,2 Å, and the diameter of the potassium ion is 2,66 Å, 18-crown-6 is more specific in relation to this metal.

Inorganic salts are generally poorly soluble in organic solvents. Thus, potassium permanganate is insoluble in benzene, but its complex with the crown ether is soluble. This feature of the complex allows to perform chemical reactions in organic media using potassium permanganate.

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Published on 15 May 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst